(1) Field of the Invention
This invention relates to a process for the preparation of a disazo pigment useful for various applications, especially, for coloring polyvinyl chloride resins.
(2) Description of the Prior Art
Disazo pigments obtained from 3,3'-dichlorobenzidine and various acetoacetanilide compounds, which do not contain any water-soluble groups, have been used widely for the coloration of polyvinyl chloride resins, printing inks, paints and rubber. Their use, however, is accompanied by a drawback that when they are employed for example to color a polyvinyl chloride resin, the color of the resulting polyvinyl chloride resin varies depending on the heating and kneading conditions for the polyvinyl chloride resin even if the same azo pigment is used. The cause of the drawback has been found to reside in the heating and kneading conditions.
Accordingly, the above-mentioned drawback might be solved if exactly the same temperature conditions were employed upon heating and kneading. As a matter or fact, it is however almost impossible to make temperature conditions constant. Variations of about 5.degree.-10.degree. C. in processing temperature are unavoidable in general. When a conventional disazo pigment is employed as a colorant, the colored product unavoidably develops a color difference even if a disazo pigment prepared in the same lot is used. When the above disazo compounds are used in printing inks, they are accompanied by a drawback that the disazo pigments lose their coloring power due to their heating upon flushing. With a view toward minimizing such drawbacks, certain processes have been proposed, for example, in Japanese Patent Publication Nos. 11026/1970 and 49087/1980. However, these processes cannot sufficiently improve the heat resistance of disazo pigments upon coloration of polyvinyl chloride resins and the improvements to the heat resistance upon flushing are hardly satisfactory. When employed in gravure inks, a further problem arises that the souring resistance becomes poor.